This invention relates to a brake servo booster, primarily for use in a vehicle hydraulic braking system for the purpose of providing power assistance in the actuation of a brake master cylinder, and in which the supply of air to an integral or remote air actuated servo piston, usually in the form of a diaphragm assembly, is controlled by a valve mechanism which includes a reciprocable valve element operatively connected to a force input member and carrying an abutment member which engages a fixed part of the booster to set a retracted position of the valve element.
It is known to provide a brake servo booster with an electrical switch which is actuated by a component of the booster to control, for example, the switching of a vehicle stop light. One such prior proposal is described in U.S. Pat. No. 4718324 and incorporates a switch at the outer end of an axial extension of the booster housing, the switch being actuated to change its operative condition in response to inward movement of a force input rod. A disadvantage of such an arrangement is that the accommodation of the switch and its actuating mechanism requires a significant rearward increase in the length of the housing, which can give rise to installation difficulties when space in the axial dimension is limited.